Comparison of Deep Inelastic Scattering with Photoproduction Interactions at HERA

Photon-proton ($\gamma p$) interactions with~$Q^2<10^{-2}$~GeV$^2$ and deep-inelastic scattering ($\gamma^* p$) interactions with photon virtualities $Q^2>5$~GeV$^2$ are studied at the high energy electron-proton collider HERA. The transverse energy flow and relative rates of large rapidity gap events are compared in the two event samples. The observed similarity between $\gamma p$ and $\gamma^* p$ interactions can be understood in a picture where the photon develops as a hadronic object. The transverse energy density measured in the central region of the collision, at $\eta^*=0$ in the $\gamma^* p$ centre of mass frame, is compared with data from hadron-hadron interactions as function of the CMS energy of the collision.